But, it does not work … If I see in console console.log(quiz.currentQuestionIndex); I see the current Question but I don’t know how to use it to target the current dot and add the class active.
here is my code : https://codepen.io/Alanes/pen/vjjpwR?editors=1111

Well, your id for buttons is guess0 for example first question. For first dot is 0. You need to make where you say:
var dotId = $(".btn–default").attr(‘id’);
var dotIdLen = dotId.length;
var btnId = dotId .substring((dotIdLen-1),(dotIdLen));
$("#"+btnId).addClass(“active”);

First you gotta get your button id. Second, you determine said id’s length(example: guess0 have 6 letters length),
third take last letter by using substring method. You need to take last letter since its represent your id in dot.
Forth, used acquired id to set active class on a element with said id.
PS: Don’t forget to remove class from previous element …

Thank you for the answers!
I could go forwards with this project. But because this is OPP JS . I don’t really know how to do a reset button on this Quiz . I know I have to reset every HTML components that was created by var QuizUI = { } . You can see my code better here: https://codepen.io/Alanes/pen/vjjpwR?editors=0111